Imaging process employing friction charging in the presence of an electrically insulating developer liquid

ABSTRACT

Friction charging of insulating and photoconductive insulating members is accomplished in the presence of an insulating liquid. In a preferred embodiment, the insulating liquid is a developer liquid and the friction charging step may simultaneously erase a previously developed image.

United States Patent Goffe Sept. 23, 1975 [5 IMAGING PROCESS EMPLOYING 2,860,048 11/1958 Deubner 3 96/1 R FRICTION CHARGING IN TIIE PRESENCE 3 1 a ov OF AN ELECTRICAL INSULATING 3,427,242 2/1969 Mihajlov 96/1.3 x DEVELOPER LIQUID 3,485,738 12/1969 Carreira 96/1.3 x 75 Inventor: William L. Goffe, Webster, NY. 375411885 1970 1 96/1 LY 3,546,545 12/1970 Sato et al. 96/1 0 x [73] Assignee: Xerox Corporation, Stamford, 3,549,962 12/1970 Roth et a1. 96/1 C X Conn. 3,586,615 6/1971 Carreira 96/l.3 x

, 3,717,461 2/1973 1161116 96/1 LY 1 Flledl 14, 1973 3,725,059 4/1973 Komp 96/1 LY x 3,748,127 7/1973 Amidon et a1 96/1 LY Appl. No.: 341,217

Related U.S. Application Data Continuation-impart of Ser. No. 863,664, Oct.' 3, 1969, abandoned, which is a continuation-in-part of Ser. No. 553,438, May 27, 1966, abandoned.

U.S. Cl. 96/l.3; 96/1 LY; 96/1 C; 117/37 LE Int. Cl. G03C 13/02; G036 13/10 Field of Search 96/1 LY, 1 C, 1.3; 117/37 LE References Cited UNITED STATES PATENTS 10/1942 Carlson 96/] R Primary Examiner-Norman G. Torchin Assistant Examiner-John R. Miller Attorney, Agent, or Firm-James J. Ralabate; David C. Petre; Richard A. Tomlin [5 7] ABSTRACT Friction charging of insulating and photoconductive insulating members is accomplished in the presence of an insulating liquid. In a preferred embodiment, the insulating liquid is a developer liquid and the friction charging step may simultaneously erase a previously developed image.

15 Claims, 3 Drawing Figures IMAGING PROCESS EMPLOYING FRICTION CHARGING IN THE PRESENCE OF AN ELECTRICALLY INSULATING DEVELOPER LIQUID BACKGROUND OF THE INVENTION This application is a continuation-in-part of my copending application Ser. No. 863,664, filed Oct. 3, 1969, which was a continuation-in-part of my application Ser. No. 553,438, filed May 27, 1966 both now abandoned.

This invention relates in general to imaging and in particular to electrography, including xerography, using liquid developers.

The use of electrostatic charge patterns to form images is well-known in the art. Probably the more common process in which electrostatic charge patterns are used to form images is xerography. In the art of xerography, exemplified by Carlson, U.S. Pat. No. 2,297,691, it is usual to employ the simultaneous application of electric field and a pattern of activating radiation on a photoconductive insulating member to form an electrostatic charge pattern otherwise known as an electrostatic latent image. This electrostatic latent image then is capable of being utilized such as, for example, by the deposition of electroscopic material thereon to form a visible image. Usually, the order of procedure is to sensitize the xerographic plate by applying a uniform charge to the surface of the photoconductive member after which exposure is made. Sensiti zation may be accomplished by any of various means, such as, forexample, frictional means as disclosed in the Carlson patent or ion charging means as shown in Carlson U.S. Pat. No. 2,588,699.

Other techniques for forming electrostatic charge patterns are known. For example, in U.S. Pat. No. 3,5 18,081 to Bickmore and Goffe an imagewise modification is made to a surface to alter imagewise the capa bility of the surface to accept charge. By triboelectrically charging such a surface, a charge pattern is formed which may be developed by contacting the surface with colored electroscopic marking particles which are attracted to the charge pattern.

Another broad general branch of electrostatography is generally referred to as electrography and it is considered distinct from the xerographic branch in that is does not employ a photoconductor and light exposure to control the formation of its latent electrostatic charge pattern. Electrography as it is generally known today may be divided into two broad sectors which are, xeroprinting and TESI recording. Xeroprinting may be described as the electrostatic analog of ordinary printing. This process, which is more fully described in U.S. Pat. No. 2,576,047 to Schaffert, employs a xeroprinting plate made up of a pattern of insulating material on a conductive backing so that when the xeroprinting plate is charged, as with a corona discharge electrode, an electrostatic charge pattern is retained only on the pat terned insulating sections of the plate. Image development is by the same techniques employed in xerography.

The common feature of all of these electrostatographic systems is that they employ the lines of force from the electric field of a latent electrostatic image to control the deposition of colored finely divided marking particles known as toner, thus forming a visible image corresponding to the charge pattern. The colored finely divided marking particles may be in dry powder form or may be suspended in an insulating liquid. The use of insulating liquids containing developer particles is well-known to the art. See, for example, U.S. Pat. No. 3,053,688 to Grieg.

The above techniques are useful for forming hard copy or for displays. Where it is desired to reuse the member on which the image is formed, it is necessary to remove the image, clean the member, and recharge the member, either imagewise, or uniformly with subsequent imagewise dissipation of the charge.

In addition to the use of marking particles to make charge patterns visible, other methods are known. In one such method described in U.S. Pat. No. 3,196,010 to Goffe et al. a thin, from about to about millimicrons, layer of an insulating developer" liquid is formed over the charge pattern. The electrostatic image creates interference or light scattering patterns in the liquid which are visible. The liquid may also contain a wax or dissolved material which will form a more permanent solid image as taught in U.S. Pat. No. 3,196,010 to W. L. Goffe et al., issued July 20, 1965, the disclosure of which is incorporated herein by reference.

A further technique for making electrostatic latent images visible is by using liquid crystal materials. The electrostatic field causes phase changes or changes in optical polarization which appear as differences in color or opacity. The use of liquid crystal materials to develop electric fields is shown in U.S. Pat. No. 3,642,348, issued Feb. 15, 1972, and U.S. Pat. No. 3,707,322, issued Dec. 26, 1972, which patents are assigned to the assignee of this application and which disclosures are incorporated herein by reference.

The techniques described in the last two paragraphs above are particularly useful for image display purposes. Again, in order to reuse the member on which the image is formed, it is usually necessary to remove the image, clean the member, and form a new charge pattern on the member. Normally, the above processes are carried out using ion charging means such as a corona discharge or by contacting the free surface of the developing liquid with a second electrode and applying a potential difference between the photoconductive member and the second electrode. These techniques, although successful, require expensive and bulky equipment, and high potentials. Further, usually several processing steps are required to form a charge pattern, develop the image, remove the image, clean the member on which the image is formed and to reform the charge pattern.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a system for charging an insulating or photoconductive insulating member which overcomes the above noted disadvantages.

It is another object of this invention to provide a simplified display system.

It is another object of this invention to provide a system for the substantially simultaneous cleaning and charging of an insulating or photoconductive insulating member.

It is another object of this invention to provide a system for substantially simultaneously charging, cleaning and applying a developing liquid to an insulating or photoconductive insulating'liquid.

It is another object of this invention toprovide" an" image display system which does not require a power supply.

The foregoing objects and others are accomplished in accordance with this invention by a system comprising p'roviding'an'insulating or photoconductive insulating member andcharging the member by friction charging in the presence of an insulating liquid. Friction charging the member in the presence of a liquid simultaneously cleans and charges the member. In a preferred embodiment the insulating liquid is also capable of making an electrostatic latent image visible in which case; the liquid is referred to as a developing liquid. The phrase developing liquid is intended to include insulating liquids containing particles which will migrate to charged or uncharged areas of the image or both; liquids capable of forming interference or light scattering patterns inresponse to electrical field whether they'include suspended or dissolved solids; liquid crystal materials; and liquids'containing dissolved or suspended materials which deform in response to electric field when in layer-form. Where friction charging is carried out in the presence of a developing liquid, the process is capable of charging, cleaning, and developing an image in one step.'

Where a photoconductive insulating material is friction charged in the presence of a liquid, an improvement in resultscan often be obtained by overcoating the photoconductive member with a thin insulating material. The thin overcoating may be selected on the basis of its triboelectric charge acceptance capability, that is by how high a charge can be formed on the material by friction charging. Selection of different overlay materials or rubbing materials can control the sign of'the charge placed on the overcoating. The overcoating is used-in a thickness which will not interfere with the operation of the photoconductor and may itself be photoconductive in which case a thicker layer may be used. Alternatively, the overcoating may be formed on the photoconductive member in a screen pattern. Friction charging in the presence of an insulating liquid in this'case provides a screen pattern charge.

In a further embodiment, an imagewise pattern of an insulating material: may be formed on a conductive substrate. Friction charging in the presence of a liquid will here form a charge on the insulating surface which can be developed by the techniques disclosed herein, In this embodiment a particularly useful system employs parti-" cles which deposit on the insulating pattern and are then transferred to an image receiving surface. This system is particularly useful for duplicating. Friction charging in the presence of an insulating liquid containing developer particles, charges the plate, cleans the plate and applies developer materials simultaneously.

The advantages of this improved method of charging an insulating o'r photoconductive insulating member will become apparent upon consideration of the following detailed disclosure of the invention. The best results are obtained by using a selenium xerographic plate as the photoconductive insulating member because the selenium xerographic plate has excellent sensitivity and the ability to receive and retain'a uniform charge with comparatively low dark decay which is particularly important for a reusable photoconductor display system.

4 taining' appr oximately 143 grams of PKDA 8500, a phenoxy=resin available from the Union Carbide Co.; l0.0*gr'amsof 1% Rhodamine B, an aminophenylxanthene dye in ethanol; 14. 1 grams of TO-l920, 2,5-bis- (p-aminophenyD-l,3,4-oxadiazole; 100.0 grams of cyclohexanol; 28.5 grams of Uformite 200 E, a melamine formaldehyde resin "available from Rohm and Haas. The SOP is rubbed on as aliquid and allowed to dry. The SOP, being itself photoconductive, may be placed in any desired thickness on the photoconductive mem-- ber. The toner image, when developed, is found to be substantially as sharp as images obtained on the same selenium xerographic plate with corona charging. The SOP-coated plate is also found to be abrasion resistant. Rubbing afterimag'e development in the presence of an insulating liquid, recharges the surface of the SOP to the same high, uniform surface charge obtained initially. 1

Any suitable insulating member may be used. Typical photoconductive insulating materials include inorganic photoconductors, such as zinc-oxide, cadmium sulfide, zinc sulfide,'lead'sulfide,'cadmium selenide, selenium, lead iodide, lead chromate, and mixtures thereof; organic photoconductors such as triphenyl amine; 2,4- bis-( 4,4 -diethyl-am inophenyl )-l 3 ,4-oxadiazole; N- isopropyl carbazole; triphenyl pyrrol; 4,5- diphenylimidazolidinone; 1,4-dicyano naphthalene; 2-mercapto-benz thiazole, 2,4-diphenyl-quinazoline; 5-benzideneamino-ace'naphthalene, and mixtures thereof. These materials may be used as the photoconductive layer by themselves as a one-phase photoconductor or in a'suitable' binder.

Typical binders are selenium, polystyrene resins, silicone resins, acrylic and methacrylic polymers'and copolymers and mixtures thereof. Typical nonphotoconductive insulating'members known in the art are: non-self-supporting or self-supporting films of organic resins, plastics, binders including cellulose, cellulosic materials, and'insulating resins such as lacquer coatings, and resin films and layers including urea and melamine-type resins, vinyl 'resins, acrylic resins and mixtures thereof. r

The thin overcoating 'may also comprise any suitable material which can be readily coated on the surface of The selenium xerographic plate is'preferably coated with a special organic photoconductor, referred to hereafter, as SOP. SOP is formed from a solution con,-

the insulating or photoconductive insulating member. Important requirements for the thin overcoating are that the material be abrasion resistant or replenishable and have the desired triboelectric properties. Materials having the desired triboelectric properties may be selected from a triboelectric series. For example, if it is desired to place a positive charge on the surface of the thin overcoating, the overcoating'material will be selected from those materials positioned relatively high on the triboelectric series and rubbed with a material positioned below it on the triboelectric series. Conversely, it is desired to place a negative charge on the surface of the thin overlay, the thin overlay material may be selected from those materials positioned relatively low on the triboelectric series and rubbed with a material. positioned above the overlay material on the triboelectric series. Typical materials are: Kodak Photoresist (available from the Eastman Kodak Company of Rochester,New York), which may be described generally as the cinnamate esters of polyvinyl alcohol and of cellulose, SOP, non-self-supporting or selfsupporting films or organic resins,-plastics, binders including cellulose, cellulosic materials, and insulating ers including urea andnmelamine-type resins, vinyl resins, acrylic resins and mixtures thereof. The thin overcoating may also compriseahy of the photoconductive insulators or photoconductiveinsulators in a binder or mixtures thereof listed .above. The, thickness pf the thin overcoating may be so thin as to be monomolecular. Alternatively, thethin overlay may be placed on the surface of the photoconductive insulating rnember in any thickness so long as the thin overlay does not substantially interfere with the desired properties of the photoconductive insulating rnember. v Where the insulating layer is formed in an imagewise pattern on a conductive substrate, 'it may be present in desirable thickness. f i f, I i

The insulating or developing liquid should not affect the material which is being charged or the friction charging material itself The insulating or developing liquid may be any suitable material whichwill not discharge an electrostatic imagel'Typical materials known in the art are: Sohio Odorless Solvent 3440, a mixture of kerosene fractions, silicone oils such as Dow Corning 200 Silicone Oil, 0.65 centistoke, l. O centistoke, SO'centistoke and l-c'entistoke, minera I oil, decane, dodecane, N-tetradecane, molten paraffin; and Narcoil 40 diffusion pump oil, a di-nonyl phthal ate ester available from the National Research Corporation, developing liquids such as those disclosed in US. Pat. No. 3,053,688 to Grieg andmi xtures thereof.

The friction 'chargingmaterial maycomprise any suitable material having-the desired triboelectric properties selected from the triboelectric series in accordance with its relation in the triboelectric series to the material used as the insulating or photoconductive insulating material; Typical materials known in,=the *art are: rubber, fur, wool, rayon, felt, hair, velvet; woven or knit cotton, or other suitable charging materials. The thin overcoating may be coated on the surface of an insulating photoconductor in a screen pattern to improve solid area coverage in xerographic processes.

The advantages of this improved method of charging an insulating member, will become apparent upon consideration of the detailed disclosu,e of the invention, especially when taken in conjunction ,with the accompanying drawings wherein: i g

FIGS, 1, 2 and 3 show a succession of process steps which provide an'imaging system in accordancewith this invention. j j

Referring now to FIG. l, there is seen a selenium xerographic plate comprising, feiexample, an 80 micron layer 1 of photoconductive selenium "on transparent conductive substrate 3 whichmay be, for exa'mple, NESA glass (a tin oxide coated glass). The xerographic plate is overcoated with a thin layer 5 of SOP defined earlier in the disclosure. Brush 7 having, for example, rayon fibers wet with developer liquid 9 containing marking particles 13 is rotated rapidly and caused to traverse layer 5 in the dark. 'Examplesof such developing liquids are described, for example, inIULS; Pat. No;

3,053,688 to Grieg. On completion of brush 7 traverse of layer 5, a thin layer 9 of developer liquid which contains electroscopic marking particles 13- is found on layer 5. The charging also providesa high potential on surfaceS. i it Referring now to FIG. 2., thehighly 'charge'd xeroedi ation '1 1V which allows charges to dissipateto'ground,

caused by the charge'ipattern or a change in optical po-- leaving acharge' pattern on layer 5. It should be pointed out here that the imagewise exposure 11 may also occurtthrpugh'layer 9 and 5 and may also be simultaneous withithe friction charging. Also, the liquidcould be applied to the plate prior to brush traverse.

Referring now to. FIG, .3, t here is .seen particulate image area 13 here greatly exaggerated which comprises electroscopic particles attracted from liquid layer 9. Where thetimage is to be transferred, the liquid may beremoved by evaporation or other convenient method, such as. absorption or allowing it to run off; It

should bepointed outrthat the marking particles 13 are attracted to areas of large potential difference and therefore tend to develop edge effects? preferentially producinghigh .quality line copy. For .high quality broad area coverage, itis necessary only to provide layer, 5 in, .a screenupattern which will initially provide afscreen pattern charge providing edge effects throughout irnage areas. Wherea screen pattern charge isdesired, thescreen pattern may be applied, for example, using a grav ure roller. The resulting pattern should have or more spots of overlay per inch. The advantages of sdreen pattern charging as wellknown in the an, See, f r example, Us; Pat. No. 2,598,732 to Walkup. H

Particulate image 13 may be fixed in place or transferred and fixed by conventional xerographic techniques, if desired; however, the system is particularly useful as a display since, all thatis necessary to form a new imag'e is to traverse layer 5 in the dark with brush 7a nd re expose to an image. The liquid used for a display system would be selected to be relatively nonvolatile. f u In additionjto the use of a developing liquid contain in'g marking particles to instantaneously make electrostatic images visible upon application of the developer liquid, a riurnber'of other techniques can be used. For example, whe're the insulating developer liquid used for friction charging forms a layer on the photocon'ductor of thickness in the range of from about to about 125 millimicrons, the electrostatic image creates interference colors or light scattering patterns in the liquid which arevisible. In this connection, the liquid may be acombination of a wax and solvent or a solid material which can be converted to a liquid material. The use of these films to develop electrostatic images is set out in US. Pat.N0. '31l96,0l0.

Further, the electrostatic image may be made visible by using a soft solid material which can be deformed by the electrostatiepattem.These-materials can be ap pliedin solution'or suspension form while charging as described above. The image is again readily removed by frictionally contacting the image in the presence of a solvent or suspending medium. Again, the photoconductive layer may be overcoated uniformly or in a screen pattern. v I i i A further technique is to use an insulating developer liquid which contains or'is made up of a liquid crystal material. The use of liquid crystal materials to develop or make visible electrostatic charge patterns is wellknown and depends, =forexample, on phase changes larizationfcaused by the charge pattern which result in coloror opacity differencesain' theliquid crystal mategraphic member: having a layer f developer liquid; thereon is then exposed to an imagewise pattern-,0f-r; a

rial.-=The use of both cholesteric and nematic liquid crystals is contemplated. Patents showing the use of liquid crystals to develop or make visible electrical fields are, for example, US. Pat. No. 3,642,348 and US. Pat. No. 3,707,322.

The following examples further specifically illustrate various preferred embodiments of the improved friction charging method.=The parts and percentages are by weight unless otherwise indicated.

EXAMPLE I A selenium xerographic plate comprising an 80 micron selenium photoconductive layer on a NESA glass substrate is coated with a layer of Kodak Photoresist (KPR). The KPR is rubbed on with a cloth material and allowed to dry providing about a /2 micron layer. The KPR is then charged by rubbing with a rayon cleaning absorbent soaked in a developer composition comprising carbon black pigmented polystyrene suspended in Sohio Odorless Solvent 3440. A surface charge potential of about +500 volts is generated. The coated selenium xerographic plate with the developer liquid still present on it is exposed to an image. The suspended particles migrate forming an image on the surface of the KPR. The plate is then rubbed again with the developer soaked rayon 'which cleans the plate, recharges the plate and applies developer preparing the plate for a second imaging cycle.

EXAMPLE II The experiment of Example I is repeated except that the developer liquid is provided as follows. A Staybelite resin is dissolved in heated Sohio Odorless Solvent 3440. The solution is allowed to cool which causes some of the resin to precipitate forming a white powder in suspension. Rubbing here causes a charge of over +1000 volts to be generated. High quality images are formed as in Example I except that in this Example, white images are formed. By rubbing again in the presence of the developer liquid, the plate is cleaned, recharged, and is used to form a second image of high quality.

EXAMPLE III EXAMPLE IV The experiment of Example III is repeated except that the developer liquid is replaced by a developer liquid made up of fine particles of Staybelite suspended in Sohio. Again, high quality images are prepared.

EXAMPLE v The coated plate of Example I is charged by rubbing with rayon soaked in Dow Corning 200 Silicone Oil 10.0 centistoke containing carbon black pigmented polystyrene, while the plate is exposed to .an image through the NESA glass plate providing a black particulate image corresponding to unexposed areas of the photoconductor. Rerubbing during exposure to a second image forms a corresponding second image.

EXAMPLE VI An uncoated selenium xerographic plate is charged as in Example V, except the liquid is 1.0 centistoke oil containing carbon black pigmented polystyrene particles. Images are formed as in Example V, but complementary thereto.

EXAMPLE VII The coated plate of Example I is charged by rubbing with rayon soaked in Dow Corning 200 Silicone Oil, 0.65 centistoke containing the particles of Example VI. Images are formed as in Example V.

EXAMPLE VIII The experiment of Example VII is repeated using Narcoil 4O diffusion pump oil inplace of the silicone oil. High quality images are again formed.

EXAMPLE IX The coated plate of Example I is charged by rubbing with rayon soaked in Sohio providing a uniform potential of over +1000 volts.

EXAMPLE X The SOP coated plate of Example III is charged by rubbing with rayon soaked with Dow Corning 200 Silicone Oil.l.0 centistoke providing a potential of about +1000 volts.

EXAMPLE XI The experiment of Example X is repeated using a rayon cloth soaked with Sohio providing a potential of about +800 volts.

EXAMPLE xii An uncoated selenium surface is charged as in Example XI providing a potential of about 'l000 volts.

Any of the charged members of Example IX through XII may be used for providing an image by exposing the member to a pattern of light and developing using preferably liquid developers. Rechargingas in Examples IX through XII cleans and recharges simultaneously.

EXAMPLE 'xm A 50 micron selenium film formed on the conductive surface of a NESA glass plate is rubbed vigorously with a tissue soaked in a dilute solution of ceresin wax in trichloroethylene while the selenium is exposed to an image through the NESA glass. After a few passes with the soaked tissue, a visible imagewise change appears in the interference colors associated with the thin film formed. The imagewise change in colors indicates a thicker wax coating over the unexposed highly charged areas at the edges of the charge pattern. The image thus formed is relatively more permanent than a liquid image being of a solid material. The solid image is readily removed by rerubbing with a tissue wet with a solvent or rerubbing with; the original dilute waxsolvent developer solution. Rerubbing removes the image,.-cleans.the photoconductor, recharges the photoconductor; and "develops a second image projected through the" NESA glass.

EXAMPLE XIV An imagewise pattern of Mylar polyester film is formed on a conductive substrate. The film is charged by rubbing with a developing liquid which is made up of carbon black pigmented polystyrene in Dow Corning Silicone Oil 50.0 centistoke using absorbent rayon. A black particulate image corresponding to the charged resin areas is formed. The image is transferred and the process repeated providing a second image.

Although in the above Examples, rubbing is used to impart surface charge, any method of contact charging may be used, for example, touching, impacting, rolling or twisting are suitable methods.

Although specific components and proportions have been stated in the above description of preferred embodiments of the invention, other typical materials as listed above, if suitable, may be used with similar results. In addition, other materials may be added to the system to synergize, enhance or otherwise modify the properties of the various layers or liquid developers, for example, charge transfer sensitizers may be added to the thin overcoating to assist in the light discharge of the surface potential.

What is claimed is:

l. A method for forming a visible image comprising the steps of:

a. providing a photoconductive insulating layer on an electrically conductive substrate;

b. friction charging said photoconductive insulating layer with a second material in the presence of an electrically insulating developer liquid until an electrostatic charge is produced on said photoconductive insulating layer, said second material differing in triboelectric character from said photoconductive insulating layer; and

c. exposing said photoconductive insulating layer to an imagewise pattern of radiation to which the layer is sensitive whereby a visible image is formed.

2. The method as defined in claim 1 wherein step (c) is carried out simultaneously with, or subsequent to, performing step (b).

3. The method as defined in claim 2 wherein said photoconductive insulating layer is overcoated with an electrically insulating third material, said third material differing in triboelectric character from said second material.

4. The method as defined in claim 2 wherein said photoconductive insulating layer is overcoated with an electrically insulating material in a screen pattern, said third material differing in triboelectric character from said second material.

5. The method as defined in claim 2 and further including performing steps (b) and (c) at least one additional time, wherein step (b) also removes the previously developed image.

6. The method as defined in claim 2 wherein said electrically insulating developer liquid forms a thin film on said photoconductive insulating layer and said image is an interference pattern.

7. The method as defined in claim 2 wherein said electrically insulating developer liquid contains marking particles and said image comprises deposited marking particles.

8. The method as defined in claim 2 wherein said electrically insulating developer liquid contains a material which forms a solid thin film on said photoconductive insulating layer and said image is an interference pattern.

9. The method as defined in claim 2 wherein said electrically insulating developer liquid comprises a liquid crystal material and said image comprises a difference in color or opacity.

10. A method for forming a visible image comprising the steps of:

a. providing an imagewise electrically insulating layer on an electrically conductive substrate; and

b. friction charging said electrically insulating layer with a second material in the presence of an electrically insulating developer or liquid, said second material differing in triboelectric character from said electrically insulating layer whereby a visible image is formed.

11. The method as defined in claim 2 and further including performing step (b) at least one additional time, wherein step (b) also removes the previously developed image.

12. The method as defined in claim 2 wherein said electrically insulating developer liquid forms a thin film on said insulating layer and said image is an interference pattern.

13. The method as defined in claim 2 wherein said electrically insulating developer liquid contains marking particles and said image comprises deposited marking particles.

14. The method as defined in claim 2 wherein said electrically insulating developer liquid contains a material which forms a solid thin film on said insulating layer and said image is an interference pattern.

15. The method as defined in claim 2 wherein said electrically insulating developer liquid comprises a liquid crystal material and said image comprises a difference in color or opacity.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,907,559

DATED 1 September 23, 1975 lN N William L Goffe It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 43, "disclosu e" should read mm disclosure s a- Claim 11, line 1, "2" should read 10 e,

. Claim 12, line 1, "2" should read -e l0 Claim 13, line 1, "2" should read 10 e Claim 14, line 1, "2" should read er 10 e,

. Claim 15, line 1, "2" should read we 10 w Signed and Scaled this sixth D y of January 1976 [SEAL] Attest:

Q RUTH c. MASON C.MARSHALL DANN Arresting Officer Commissioner ofParents and Trademarks 

1. A METHOD FOR FORMING A VISIBLE IMAGE COMPRISING THE STEPS OF: A. PROVIDING A PHOTOCONDUCTIVE INSULATING LAYER ON AN ELECTRICALLY CONDUCTIVE SUBSTRATE, B. FRICTION CHARGING SAID PHOTOCONDUCTIVE INSULATING LAYER WITH A SECOND MATERIAL IN THE PRESENCE OF AN ELECTRICALLY INSULATING DEVELOPER LIQUID UNTIL AN ELECTROSTATIC CHARGE IS PRODUCED ON SAID PHOTOCONDUCTIVE INSULATING LAYER, SAID SECOND MATERIAL DIFFERING IN TRIBOELECTRIC CHARACTER FROM SAID PHOTOCONDUCTIVE INSULATING LAYER, AND C. EXPOSING SAID PHOTOCONDUCTIVE INSULATING LAYER TO AN IMAGEWISE PATTERN OF RADIATION TO WHICH THE LAYER IS SENSITIVE WHEREBY A VISIBLE IMAGE IS FORMED.
 2. The method as defined in claim 1 wherein step (c) is carried out simultaneously with, or subsequent to, performing step (b).
 3. The method as defined in claim 2 wherein said photoconductive insulating layer is overcoated with an electrically insulating third material, said third material differing in triboelectric character from said second material.
 4. The method as defined in claim 2 wherein said photoconductive insulating layer is overcoated with an electrically insulating material in a screen pattern, said third material differing in triboelectric character from said second material.
 5. The method as defined in claim 2 and further including performing steps (b) and (c) at least one additional time, wherein step (b) also removes the previously developed image.
 6. The method as defined in claim 2 wherein said electrically insulating developer liquid forms a thin film on said photoconductive insulating layer and said image is an interference pattern.
 7. The method as defined in claim 2 wherein said electrically insulating developer liquid contains marking particles and said image comprises deposited marking particles.
 8. The method as defined in claim 2 wherein said electrically insulating developer liquid contains a material which forms a solid thin film on said photoconductive insulating layer and said image is an interference pattern.
 9. The method as defined in claim 2 wherein said electrically insulating developer liquid comprises a liquid crystal material and said image comprises a difference in color or opacity.
 10. A method for forming a visible image comprising the steps of: a. providing an imagewise electrically insulating layer on an electrically conductive substrate; and b. friction charging said electrically insulating layer with a second material in the presence of an electrically insulating developer or liquid, said second material differing in triboelectric character from said electrically insulating layer whereby a visible image is formed.
 11. The method as defined in claim 2 and further including performing step (b) at least one additional time, wherein step (b) also removes the previously developed image.
 12. The method as defined in claim 2 wherein said electrically insulating developer liquid forms a thin film on said insulating layer and said image is an interference pattern.
 13. The method as defined in claim 2 wherein said electrically insulating developer liquid contains marking particles and said image comprises deposited marking particles.
 14. The method as defined in claim 2 wherein said electrically insulating developer liquid contains a material which forms a solid thin film on said insulating layer and said image is an interference pattern.
 15. The method as defined in claim 2 wherein said electrically insulating developer liquid comprises a liquid crystal material and said image comprises a difference in color or opacity. 